1. Technical Field
This disclosure relates to a heat-sensitive recording material widely used in the fields of printers such as output of computers, calculators, and the like, medical measurement recorders, low-speed and high-speed facsimile machines, automatic ticket vending machines, thermal copiers, handy terminals, and POS system labels, and particularly, to a heat-sensitive recording material improved in acid resistance and water resistance to edible vinegar and the like.
2. Description of the Related Art
Conventionally, there have been various proposals for recording materials, for which provided on a substrate such as a sheet of paper or synthetic paper or a plastic film is a heat-sensitive color developing layer mainly composed of a colorless or light-colored leuco dye and a color developer that develops color of the leuco dye upon contact therewith, utilizing a color developing reaction by heat, pressure, and the like between the leuco dye and the color developer. Advantages of heat-sensitive recording materials of this type, including capability of short-time recording with a relatively simple apparatus without the necessity for applying such complicated processes as development and fixing, low noise level, and low costs, have allowed them to be used as recording materials not only for copying of books, documents, and the like, but also for electronic computers, facsimile machines, ticket vending machines, label printers, recorders, handy terminals, and the like.
As heat-sensitive recording materials, there has been a demand for materials that are capable of quick color development with high density and of imparting high rigidity to color-developed images and backgrounds. Furthermore, in recent years, heat-sensitive recording materials have come to be used in large quantities in the fields of labels, receipts, and the like where reliability of recorded images is regarded as important. Accordingly, there has been a demand for ones that offer high storage stability against water and acidic component substances contained in food and plasticizers, oils and fats, and the like contained in organic polymeric materials used for packages.
Conventionally, in order to remove such drawbacks, attempts have been made for improvement by providing a protective layer composed mainly of a water-soluble resin and a crosslinking agent on the heat-sensitive color developing layer. However, in the application of a POS label or the like, because of exposure to various opportunities for water adhesion, the heat-sensitive recording material has had a drawback that the protective layer is dissolved, image deletion and density reduction occur, and applied prints drop by water, and water resistance has had of yet been insufficient despite a slight improvement.
To overcome this problem, protective layers composed of diacetone-modified polyvinyl alcohol as the water-soluble resin and a hydrazine compound as the crosslinking agent have been proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 08-151421 and 11-314457, but each has the following drawbacks: waterproofing reaction progresses in the state of a coating solution and thus its viscosity increases with time; poor water resistance is provided; increased viscosity of a coating solution for heat-sensitive color developing layer; and inhibition of color development in the heat-sensitive color developing layer by a hydrazide compound. Moreover, protective layers composed of these materials have suffered from a problem that they undesirably dissolved particularly by impregnation with such an acidic substance as edible vinegar, whereby image disappearance and print peeling occur.